As you make your way through the automotive repair world, you will most certainly run across left hand thread. Most thread is traditional right hand thread, which means turning the nut or bolt clockwise will tighten the aforementioned object (turning counter clockwise loosens).
There are occasions where left hand thread is required. Left hand thread loosens and tightens opposite to regular right hand thread (turn clockwise to loosen left hand thread). If you have an object that is spinning, it may spin regular right hand items loose, and thus they use left hand thread.